Mismatch in a charge pump corresponds to mismatch between up and down currents and occurs due to a difference between transconductances Gm of a PMOS transistor and an NMOS transistor constructing the charge pump according to PVT (Process-Voltage-Temperature), and mismatch occurring when bias voltages of the transistors are generated.
When mismatch occurs in a charge pump included in a PLL, changes in control voltage of a voltage controlled oscillator (VCO) can cause reference spur and jitter.